It is easy to see why the prediction markets are swinging GOP. If the Democrats want to maintain House control, they had better hope for a dramatic improvement in the economy and the President's approval ratings.

from the chart for Reagan/Carter the highest point happens during the first month in the office. this makes sense since people are deluded into thinking that the new president will make a difference. two years later the crowd realizes that the president can make very little difference in their lives so by that point they hate the guy.

in general approval/disapproval should be normalized by the quality of life of the voters at the time.